Metal diaphragm assemblies have been widely employed in industrial instrumentation throughout the years as, for example, a primary element for sensing fluid pressure applied directly thereto, or as an isolation diaphragm in liquid filled systems in which the applied pressure is faithfully transmitted to some other pressure responsive member.
Regardless of the application of the diaphragm assembly, it is important for overall performance characteristics of the device that a hermetic, metallurgically sound bond be created between the diaphragm and its mounting body. In most process applications, such a bond is readily achieved by conventional techniques (e.g., fusion welding) because the diaphragm and mounting body are formed from the same metal, usually stainless steel.
However, when the pressures of many highly corrosive process fluids are to be measured, the diaphragm is usually made of tantalum or other similar chemically inert, expensive metal. To preserve manufacturing economy, it is highly desirable to retain the stainless steel mounting body in such corrosive applications.
As is well known in the art, the highly corrosion resistant metal diaphragm (e.g., tantalum) and the mounting body formed of a more chemically active metal (e.g., stainless steel) are metallurgically incompatible, i.e., the chemical and physical properties of the metals are substantially dissimilar so as not to be fusion welded by conventional techniques. Weldability as used herein refers to the formation of a sound, homogeneous, hermetic molecular bond along the point of contact between the two metals, as for example, when two metallurgically compatible molten metals are intermixed by fusion. Attempts to fusion weld incompatible materials usually result in the formation of nonhomogeneous zones of intermetallic compounds that are subject to brittleness and cracking.
The problem of providing a sound bond is further complicated by the fact that these diaphragm assemblies are sometimes required to withstand repeated temperature cycling over wide temperature extremes (e.g., -40.degree. F. to 700.degree. F.), while at the same time retaining their hermiticity. Hence, other common joining techniques such as, swaging, press fitting and bonding by plastics, adhesives or grazing alloys are unacceptable for most industrial applications.
One simple way to provide a solution to the problem is to make the mounting body of the same inert metal as the diaphragm; however, as mentioned, this approach is extremely costly. Additionally, the overall size of such matched diaphragm assemblies cannot always be reduced to suit a particular cost situation because the size of the diaphragm usually has a bearing on the accuracy, response, sensitivity and range of the instrument.
In the past, significant effort has been directed to the problem as, for example, disclosed in U.S. Pat. No. 3,675,540, which proposes affixing a thin tantalum diaphragm to a stainless steel fixture by seamwelding the peripheral edges of the diaphragm to the fixture. It is well known that seamwelding in this manner will not produce a sound, hermetically sealed bond between the dissimilar metals, but rather one which contains intermetallic compounds and is thus structurally inferior to a fusion welded joint, and ultimately subject to premature failure by cracking. As this prior patent clearly discloses, additional support for mounting the diaphragm must be provided by tightly adhering the diaphragm to mating surfaces of its back-up fixture by means of a contraction or press fit.
A more recent attempt is found in U.S. Pat. No. 4,046,010, which suggests that a tantalum diaphragm can be hermetically sealed to a stainless steel body of a pressure transducer by a form of braze-welding. The braze-weld is formed by heating a stainless steel weld ring above its melting point, but below that of the tantalum diaphragm, whereby a "weld" is formed between the stainless steel ring and the body while the molten steel flows along the outer periphery of the diaphragm to produce a type of brazed area at the surface interface between the diaphragm and the steel body. It is apparent that this solution also suffers from serious drawbacks because the actual joint interface is still a brittle one of stainless steel and tantalum. Applicants in this patent disclosure had to provide corrosion protection for the stainless steel weld by means of an elastomeric seal located on the inner circumference between the diaphragm and the body so that the process fluid could not contact and hence destroy the exposed stainless steel ring and weld.
Notwithstanding all of the prior development efforts in this field, it is apparent that the need still exists for an improved diaphragm assembly capable of joining two dissimilar metals by a sound molecular bond that offers a maximum in reliability, safety and cost savings.